Air supply and recirculation system

ABSTRACT

A method of air recirculation and air makeup for a nuclear reactor control room practiced with air supply apparatus having an alternate purification loop (84) in parallel with a portion of the recirculation loop (26). In the alternate purification mode of operation of the system a predetermined portion of the recirculated air is drawn through the purification loop (84) in parallel with a flow of the remainder of the recirculated air through the normal parallel recirculation flow path (86). The volume of air drawn through the purification path (84) is established to maintain the purification loop operating at its maximum efficiency. A makeup air supply loop (24) having a normal flow path (42) and an alternate, parallel filtration flow path (50), communicates with the recirculation loop (26). The amount of air inputted to the makeup loop (24), supplied through the makeup loop (24) to the recirculation loop (26), and drawn through the filtration path (50) are controlled, in the alternate filtration mode of operation, to establish a counter flow of air in the normal makeup flow path (42) so that none of the makeup air supplied to the recirculation loop (26) bypasses the filtration path (50).

This application is a continuation of application Ser. No. 06/604,300filed Apr. 26, 1984, now abandoned.

BACKGROUND OF THE INVENTION

This invention pertains generally to air makeup and recirculationsystems and more particularly to such operations where purification ofthe air is required.

Air recirculation and makeup for nuclear reactor plants requirepurification flow in alternate paths to maintain the purity of a fluidwithin the plant in the event the plant becomes contaminated. Forexample, in the environmental control for a nuclear reactor powerstation control room purification flow paths are used in the unlikelyevent of an accident to protect plant personnel. Generally, there isprovided a positive supply of conditioned air to maintain positivepressure within the control room relative to the surroundingenvironment. A sustained positive pressure within the control roomreduces the unfiltered in leakage of air that is assumed would occur ifthe control room was not pressurized. Normal leakage paths consist ofcracks and seams in walls, floors, ceilings and fluid systems.

The environment in the reactor control room is normally controlled byrecirculation which draws air from the control room and recirculates itthrough an air conditioning unit designed to maintain a selectedhumidity and temperature. In addition, makeup air communicates with theair conditioning unit to makeup for normal exfiltration and sustain thedesired pressure. In the unlikely event of an accident involving therelease of contamination it is desirable to filter the incoming airpassing through the makeup system to avoid the introduction ofcontamination into the control room environment. Under suchcircumstances it is also desirable to purify the air recirculated to thecontrol room to reduce the effects of any contamination that might beotherwise introduced. If systems to effect the purification were placedin series with the normal makeup and recirculation flow, it wouldgreatly increase the required filtration capacity above practical limitsand effect their reliability. Alternatively, if a makeup filtration flowplaced in parallel with the normal makeup flow path it would requireexpensive and elaborate means for closing the normal flow path underemergency conditions to assure that no filter bypass leakage existed, sothat the efficiency of the filtration would be increased. Additionallythe control of make up air flow would have to be modulated to controlthe building pressure and thereby would not allow full flow airfiltration at the optimum capacity of the filtration operation.Recirculation through alternate parallel filters in a bypass mode wouldallow filter bypass air flow with resulting lower filtration efficiency.Accordingly, a new method of maintaining pure the air in the work areaof a nuclear-reactor plant is desired. It is desired that the makeup andcirculation of the air shall have an alternate purification capabilitythat minimizes the capacity of the purification required whilemaximizing efficiency.

SUMMARY OF THE INVENTION

Briefly, this invention enhances air supply of the prior art thatrequire an alternate air conditioning mode of operation by providing analternate air conditioning flow in an arrangement that assures that, inthe alternate mode of operation, all of the air passes through theconditioning flow path without any leakage through the normal flow path.

In accordance with this invention there is provided a method formaintaining the work area in a nuclear reactor plant pure and at apositive pressure over the air outside of the work area. In the practiceof this method the air is recirculated through the work area throughrecirculating conduits which includes an air purification unit. Makeupair is supplied from the outside to the work area through an additionalconduit and also through an air filter in parallel with the additionalconduit. The air is pumped through the air filter at a higher rate thatthe rate at which it is being supplied from the outside. The makeup airis thus prevented from bypassing the air filter.

In the practice of this invention, the conduit which supplies the makeupair is isolated from recirculation conduits in the event that the airoutside of the work area becomes contaminated. In addition, the flow ofmakeup air and recirculating air are each controlled in dependence uponthe air pressure within the work area.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram of a nuclear reactor power plant controlroom ventilation system including the air conditioning flow circuitstypically used in the practice of this invention;

FIG. 2 is a schematic diagram of a portion of the ventilation systemillustrated in FIG. 1, having the normal air supply and recirculationpaths shown in bold lines;

FIG. 3 is a schematic diagram of a portion of the ventilation system ofFIG. 1, having the recirculation flow path with filtration shown in boldlines;

FIG. 4 is a schematic diagram of a portion of the ventilation system ofFIG. 1, with the makeup air supply and recirculation flow paths, bothwith filtration, shown in bold lines; and

FIG. 5 is a schematic diagram of a portion of the ventilation system ofFIG. 1, having the control room air discharge flow path shown in boldlines.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention maximizes the efficiency and minimizes the requiredcapacityof air conditioning units in a air supply system where eitherair conditioning is provided in an alternate mode of operation or whereit is not required to condition the entire volume of air passing throughthe system in a single pass. The advantages and benefits of theinvention can readily be appreciated in an application to a nuclearreactor control roomventilation system. A typical system to practice themethod of this invention is illustrated in FIG. 1. The general systemillustrated includes a number of redundant subsystems to assurereliability. The basicsystem is shown in area 10 of FIG. 1, with aredundant counterpart identified generally by reference character 12.Added redundancy is supplied to the several trains 10 and 12 throughemergency alternate air intakes 14, which are respectively controlled byisolation dampers 16 and 18. Further redundancy is provided by the airintake path crossover link 20, controlled by isolation damper 22. Eachof the trains 10 and 12 includes three main subsystems; an air makeupsupply system 24, an air recirculation system 26, and a reactor controlroom purge system 28. It should be appreciated that like referencecharacters are employed in the respective redundant subsystem toidentify corresponding components.

The practice of this invention to provide control room ventilation canbe appreciated from an understanding of the operation of one of theredundantventilation trains, for example 10, shown in FIG. 1. The makeupair supply subsystem 24 includes an inlet conduit 30 having an airintake 32 at one end in series with redundant isolation dampers 34 and36 and modulation air flow control dampers 38 and 40. Modulation dampers38 and 40 provide alternate parallel flow paths for the air beingtransported through inlet conduit 30; dampers 38 and 40 beingrespectively shown as double vane and single vane flow control dampersand alternately employed, respectively, to control the rate of flow ofrelatively large or small volumes of air through the inlet conduit 30. Anormal air path conduit 42 is located in series with the parallelarrangement of modulating dampers 38 and 40 at one end and junctionterminal 48 at the other end at which point the makeup air is madeavailable to the recirculation subsystem 26. The normalair path conduit42 includes a series arrangement of redundant isolation dampers 44 and46. An alternate filtration flow conduit 50 is situated in parallel withand coupled to the normal flow path conduit 42; at one end at a junctionbetween the parallel arrangement of modulating dampers 38 and 40 and theredundant isolation dampers 44 and 46, and at the other endbetween theisolation dampers 44 and 46 and the junction 48. The filtrationflow path50 includes a series arrangement of isolation damper 56, makeup airfiltration unit 52, isolation damper 58, centrifugal fan 54, andisolation damper 60. The makeup air filtration unit is a charcoaladsorberunit, of the type generally employed in reactor control roomventilation systems, having specifications designed to satisfy theappropriate governmental regulations (i.e. U.S. Nuclear RegulatoryCommission Regulatory Guide 1.52, Revision 2, March 1978).

The recirculation subsystem 26 includes a recirculation intake conduit61 having a recirculation air inlet 62 in series with isolation damper64, axial air return fan 66, isolation damper 68 and modulation damper70; thelatter unit controlling the rate of air recirculated through thesystem. The recirculation intake conduit 61 communicates with an airreturn conduit 71 at juncture 73 just downstream of the modulationdamper 70. Theair return conduit 71 includes a series arrangement ofisolation damper 72,air conditioning unit 74, isolation damper 76,centrifugal air supply fan 78, check damper 80 and supply air outlet 82.The recirculation subsystem 26 also includes an air purification loop 84connected at one end to junction 73 and at the other end to thedownstream juncture of bypass segment 86 of return air conduit 71 on theupstream side of isolation damper 72. The air purification loopincludes, in series, isolation damper88, air purification unit 90,isolation damper 92, centrifugal fan 94, and isolation damper 96. Theair purification unit 90 is a charcoal adsorber unit similar to themakeup air filtration unit 52. A pressure sensor 98 ispositioned tosense the pressure in the control room at the intake to therecirculation system and cooperates with modulation dampers 38, 40 and70 to control the air flow rate through the makeup air supply system 24and the recirculation system 26.

The reactor control room purge discharge system 28 communicates with theair recirculation system 26 at a juncture between the modulation damper70and isolation damper 68. The discharge system includes, in series,back to back, redundant isolation dampers 100 and 102 and dischargeoutlet 104. The discharge system 28 operates, upon command, to purge airfrom the control room as will be appreciated hereafter with respect tothe description of the flow path illustrated in FIG. 5.

FIG. 2 illustrates the normal operation of ventilation train 10 in thepractice of this invention showing the normal air flow path in boldlines.In the explanation to follow, typical air flow rates will be citedto further an understanding of the practice of this invention, but therates described should not be considered as limitative of thisinvention. Duringnormal operation positive air pressure is maintainedwithin the control room to sustain the desired environment. In thisexample it is assumed that approximately 2,500 CFM of air escapes fromthe control room through normal kitchen and lavatory exhaust facilities,and that approximately 1,000 CFM escapes through other exfiltrationmechanisms. Accordingly, to sustain a desired air pressure within thecontrol room the makeup air is required to replace the air lost throughthese mechanisms.

In the normal mode of operation in the practice of this inventionmodulation damper 38 is responsive to a signal from pressure sensor 98,atthe recirculation inlet 62, to control the air intake of makeup air atinlet 32, to provide the desired volume of air replacement. In this modeof operation modulation damper 40 and isolation dampers 56, 58 and 60are closed to establish a normal air supply flow path through isolationdampers 34 and 36, modulation damper 38 and isolation dampers 44 and 46tojuncture 48, at which point the desired volume of makeup air isavailable for introduction into the recirculation system 26. The amountof air actually supplied to the control room through outlet 82, in therecirculation system 26, is determined by the capacity of the supply airfan 78 and the head that it has to draw against. This head is varied bymodulation dampers 70 and 38, with the proportion of air supplied fromtherecirculation input and the makeup air input 32 determined by thedegree ofmodulation of the respective dampers 70 and 38. In thisexample, 30,000 CFMof air is returned to the system through the supplyair outlet 82; 3,500 CFM being supplied as makeup air as aforedescribedand the balance being provided by way of the recirculation air input 62by axial fan 66. In thismode both isolation dampers 64 and 68 aremaintained in their open position, while redundant isolation dampers 100and 102 are closed to prevent loss of air through the discharge output104. The makeup air and the recirculated air join at juncture 73 forprocessing through the air conditioning unit 74 prior to being suppliedto the control room. In this manner the control room atmosphere ismaintained at the desired comfort level at a positive pressure relativeto the surrounding environment.

FIG. 3 illustrates a second mode of operation of this invention, whichcan be employed in the practice of this invention in the unlikely eventof an accident, under circumstances where the control room is isolatedfrom the outside wherein, ideally, the infiltration is low. In such anevent the makeup air supply system 24 and the purge system 28 would beisolated fromthe recirculation, by closing the corresponding isolationdampers; and the recirculation would be expanded to include filtrationpath 84, which is designed to maintain a fixed rate of flow of airthrough the air purification unit 90 so that the unit operates at itsmaximum efficiency. The amount of air flow through the filtration unitis established by the capacity of the filter fan 94, which is sized tomatch the efficiency characteristics of the purification unit 90. Inthis mode of operation airis drawn into the recirculation input 62, byway of the axial fan 66, through modulation damper 70 to juncture 73. Inthis example 32,000 CFM ofair is drawn into the system through therecirculation intake 62 and the filter fan 94 draws 4,000 CFM of thatintake through the air purification unit 90. The remainder of 28,000 CFMof air is drawn through the bypass line 86 by the supply air fan 78where it joins with the filtered air returned to conduit 71 by thefilter fan 94. The total 32,000 CFM of air is then drawn through the airconditioning unit 74 where it is conditionedand returned to the systemby way of air supply fan 78 and supply outlet 82. In this way thefiltration unit is operated at optimum efficiency witha portion of theair filtered during each recirculation pass; the amount ofair filteredbeing determined by the design requirements of the system for practicingthis invention.

An alternate emergency mode of operation in the practice of thisinvention,illustrated in FIG. 4, is designed to accommodate postaccident conditions and infiltration in the control room, which mightarise, for example, by the impact of wind on the outside of thebuilding. In this example it is assumed that there is approximately1,000 CFM of air exfiltration. In thismode of operation, the dischargesystem 28 is closed and the recirculation system 26 operates withfiltration through conduit 84, similar to the arrangement illustrated inFIG. 3. The 1,000 CFM of exfiltration is made up by the makeup subsystem24 through air intake 32. Modulation damper 38 is closed and modulationdamper 40 is opened to control the smaller volumeof air passing throughthe makeup system. In addition, the isolation dampers in filtration loop50 are opened to permit the makeup air to be filtered before beingintroduced into the recirculation at junction 48. The amount of airpassing through the filtration loop 50 is determined by the filter fan54, which is sized to draw a rate of air flow through the filtrationloop 50 which is larger than the rate of flow of air passing throughmodulation damper 40, and available at junction 48 to the recirculationsystem 26. In this manner a counter flow is established in the normalmakeup air flow which prevents any of the makeup air introducedthroughmodulation damper 40 from bypassing the filtration unit 52. The amountof air drawn through the filtration system is also designed to optimizethe efficiency of the makeup air filtration unit 52. In atypicalexample, with 1,000 CFM of makeup air available throughmodulation damper 40, approximately 1,500 CFM of air would be drawnthrough the filtration loop 50, 500 CFM of air would counter flowthrough the normal flow path 42, and 1,000 CFM of filtered air would beavailable for makeup to the recirculation at juncture 48 as illustratedin FIG. 4. In this arrangementmodulation damper 40 would limit theamount of air intake to 1,000 CFM and,together with the air flowresistance established at junction 48 by the modulation of damper 70,would assure that all of the 1,500 CFM of air drawn through filtrationloop 50 is not deposited into filtration path 84,to maintain a counterflow of 500 CFM through the normal makeup flow path 42. In this exampleapproximately 30,500 CFM of air is drawn into the recirculation airintake 62 under the influence of the air return fan 66, air supply fan78 and modulation of damper 70. The air circulating in the recirculationloop is split at juncture 73, with 3,000 CFM of air flowing to juncture48, to combine with the 1,000 CFM of air made available by themakeupsystem 24, to accommodate the capacity of filter fan 94. The remainderof 27,500 CFM of air introduced at the juncture 73 flows throughbypassleg 86 and combines with the filtered air exiting filtration loop 84atthe juncture to the input of air conditioning unit 74. The conditionedair is then returned to the system by the supply air fan 78 through theair supply outlet 82. Accordingly, the makeup air supplied to the systemreceives two stages of filtration. Thus, it can be appreciated that thedesired air flow rates through the various legs of the system arecontrolled by the judicious choice of components and by the modulationof dampers 40 and 70, which are adjusted in accordance with the pressureat the air recirculation input as identified by the pressure sensor 98.It should be further appreciated that given the aforestatedspecifications the choice of components for the practice of thisinvention is a matter ofbasic engineering design.

FIG. 5 illustrates the air flow path employed to purge air from thecontrolroom in the practice of this invention. When the purge sequenceis desired the modulation damper 70 is closed and isolation valves 100and 102 are opened. The return air fan 66 then draws the air from thecontrol room anddischarges it through the discharge outlet 104.

Referring to FIG. 1 it will be appreciated that in the event of amalfunction in ventilation train 10 ventilation train 12 can assume anyofthe aforedescribed modes of operation demanded for the practice ofthis invention. In addition, an alternate intake path exists throughisolation damper 22 in the event of a malfunction in one of the airintakes. Furtherredundancy is provided in the air intake through thealternate air intake paths which enter at inlet 14 and are controlled byisolation dampers 16 and 18. Reliability is increased by the use of twoindependent room pressure controllers, one at each of the redundantrecirculation air inputs. Though the isolation dampers are shown asmanual dampers it shouldbe appreciated that they can be replaced withmotorized dampers, well knownin the art, and controlled automatically toremotely implement the various functions of this invention heretoforedescribed. The system for practicing this invention also providesredundancy at each of the isolation dampers, with dampers being providedon either side of each of the operating units to accommodate repair ofthe units without breaching the integrity of the air flow lines.Accordingly, the invention maximizes the efficiency of the airfiltration and enhances the reliability and versatility of ventilationemploying alternate filtration.

What we claim is:
 1. The method of maintaining the air in a work area ofa nuclear-reactor plant substantially pure and at a positive pressurewith respect to the outside air, the said method comprisingrecirculating the air through said work area through recirculatingconduits including air purification means, supplying make-up air to saidwork area from an air intake through an additional conduit, alsosupplying said make-up air through air filtration means connected in aparallel flow path with said additional conduit, and pumping saidmake-up air through said filtration means at a higher rate than it isbeing supplied from said air intake to prevent said make-up air frombypassing said filtration means through said additional conduit.
 2. Themethod of claim 1 which includes the step of isolating the additionalconduits which supply the make-up air from the recirculating conduits.3. The method of claim 1 including the step of controlling the flow ofmake-up air to maintain the air in said work area at a positivepressure.
 4. The method of claim 3 including the additional step ofcontrolling the flow of recirculating air to the work area.